Entering data into a data processing device is accomplished using a data input device such as a keyboard, mouse, or joystick. Although electronic devices are constantly being miniaturized, the size of the various associated data input devices cannot be substantially decreased since they must conform to the size of the user's hands. Methods for inputting data have therefore been devised in which the user's hands do not have to touch the device. U.S. Pat. No. 5,767,842 to Korth, and U.S. Pat. No. 6,650,318 to Arnon for example, disclose an optical system in which a camera is used to monitor a user's hand and finger motions. A software application interprets these motions as operations on a physically non-existent computer keyboard or other input device. In these systems, the camera has a fixed position, so that the background of the images remains constant. This allows the software application to make use of informational present in the constant background in order to detect the user's hands in each image. This system, therefore, cannot be used in a device that in use is moved, because in this case, the background of the images is not constant, so there is no reliable background information in the images. Devices that are moved in use include hand-held devices such as a palm plot, personal digital assistant (PDA), a mobile telephone, a digital camera, and a mobile game machine.